The invention pertains generally to composite propellants and in particular to strengthening composite propellants through a binder-filler interaction.
Composite propellants are nonhomogenous propellants and comprise primarily crystaline oxidizers and metal fuels uniformally suspended in a resin binder. The crystaline oxidizers are usually inorganic compounds, such as, ammonium perchlorate or potassium nitrate but can also be organic, for example, cyclotrimethylenetrinitramine (RDX) or cyclotetramethylenetetranitramine (HMX). The metal fuels are powders of elemental metals, such as, aluminum and magnesium. In contrast with these ingredients, the binders are organic polymers such as urethanes, polyamides or vinyl polymers. Often as much as 80 weight percent of the propellant is oxidizer along about 10 weight percent of metal fuel, thus, giving a solids loading of around 90 weight percent of the total composition.
The high solids loading requires an interaction between the binder and the filler in order to prevent dewetting, i.e., separation around a particle of filler. If dewetting occurs between the binder and filler, the flame front propagates below the burning surface to produce a more rapid combustion of the propellant. This development may lead to an over pressurization in the rocket motor, and/or higher burning rate slopes, and/or shorter action times, thus, resulting in a failure of the rocket motor specification. Another problem resulting from dewetting is that these separations or gaps act as focal points for crack propagation. Such a failure could easily cause detonation because of the sudden increase in burning surface area and the resulting increase in gas production. The gaps around oxidizer crystals can lead to blanching which results in crazing on the surface. Blanching adversely affects performance consistency.
A poor binder-filler interaction also causes problems for a missile in storage with time. If the motor is not sealed or if the seal leaks, moisture can more readily penetrate the propellant when dewetting has occurred. This can lead to a degradation of the binder, grain deformation and/or grain cracking when motors are temperature cycled. This problem is especially serious for nitramine oxidizers, such as, RDX and HMX. When these oxidizers degrade, nitrogen oxides are produced which catalyze cross-linking in the binders to produce an extremely stiff and brittle binder. With ammonium perchlorate (AP) oxidizers, moisture may dissolve the oxidizers to form an acid which also attacks the binders and/or cause the oxidizer to bloom or grow uncoated crystals on the bore. The resulting binder has a poor strength capability. The binder is not able to cycle to cold temperatures and the binder has a tendency to break down, causing a slump at the bore and thus a high pressure.
Poor interaction between binder and filler is found mostly with oxidizers and not with the metal fuels. The metals are not a problem because the metals have a more irregular surface and a greater ease for chemical bonding with the filler. On the other hand, oxidizers have surfaces which are very smooth, and in some cases the oxidizers do not chemically bond with the filler.
Attempts to improve the binder-filler interaction in composite propellants have included the addition of a re-inforcing agent, a wetting agent or a bonding agent. A re-inforcing agent increases the chemical and physical properties of the binder around the oxidizers, often producing hardened encasements for the oxidizer crystals. The disavantages of these agents are often a slight deteriation in the processability of the propellant and a minimal effect in reducing the problems caused by poor binder-filler intereaction. A wetting agent acts to wet the solids, thereby reducing the separations between the binder and filler. The improvement in binder quality when produced, is too slight to be of much consequence. A bonding agent produces an intereaction between the oxidizer crystal and the binder by forming either primary or secondary bonds with the oxidizer and a primary bond with the binder.
Since the ideal additive is a bonding agent, most research has been conducted on developing this additives, but due to the many requirements, few bonding agents have been developed. especially for HMX or RDX. In addition to forming a bond between the very chemically dissimilar crystal and binder, a bonding agent should also be chemically and thermally stable. A bonding agent should also be compatable with all oxidizers and binder systems. Preferably the bonding agent should also perform other functions, such as acting as a wetting agent or a scavanger for soluble metal impurities.
The presently used bonding agents, except for the bonding agent disclosed in U.S. patent application Ser. No. 745,519, filed 29 Nov. 1976 by John Consaga, are not universal bonding agents, that is, they can only be used for one or a few binder-filler systems but not for all binder-filler systems. One agent is N,N biscyanoethyl-2,3-dihydroxypropylamine. This agent cannot be used with nitramine oxidizers, and good results are only obtained with ammonium perchlorate systems. The binder systems in which this agent can be used are polyurethane systems with polyether and isocynate cures. With filler systems, such as the important hydroxyl-terminated polybutadiene binder, the bonding agent has compatibility and solubility problems. The performance of this agent in a polyurethane-AP system is barely adequate.
Another bonding agent is the addition product of tris[-(2-methyl) -aziridinyl]phosphine oxide and tartaric acid and adipic acid. This bonding agent is susceptable to moisture degradation and the bond is barely adequate. Furthermore, the addition adduct is polyfunctional and tends to homopolymerize on standing.
The bonding agent, bis[1-(2-methyl)-aziridinyl]benzene-1,3 dicarboxylic acid amide, has problems with moisture and poor bonding due to a lack of interaction with the binder network. In addition, this bonding agent can only be used for ammonium perchlorate oxidizers.
The only universal bonding agent is the one disclosed in the above patent application by John Consaga. The major difficulty with this bonding agent is the difficulty in processing the propellant at solid loadings of 88% and higher.